The transportation of natural gas from the supply location to the tanks at the market by ship or truck transportation tanks requires that the gas be highly compressed to make the transportation economic. The expense of high pressure transportation tanks (e.g. 3000 p.s.i.) rather than at atmospheric pressure (e.g. 0 p.s.i.) is more than offset by the fact that about 250 times as much gas product can be transported.
A second problem exists that if the tanks at the market have an intermediate pressure such as 600 p.s.i. When the 3000 p.s.i. high pressure transportation tanks are dumped into the market tanks, approximately 1780 p.s.i. will remain in the transportation tanks. This means that approximately 60% of the product transported remains undelivered.
Two choices have remained here in the art. First, you can simply leave the gas in the transportation tanks for the return trip and always be transporting this 60% of the volume back and forth from the supply location to the market location. Secondly you can provide gas compression pumps to pump the stranded gas from the ship or truck transportation tanks and deliver all the gas to market. The gas compressors are expensive and expensive to operate. However, the higher cost in many cases is the time tying up the access to the terminal while they are being pumped out. Especially in the case of ocean going ship terminals, the dock time is an expensive charge. However, because of the efficiency of the compressors, residual pressure never comes below about 600 p.s.i. or 20% of the original pressure.
Throughout the history of the transportation of natural gas, the balance between the transportation of the stranded gas in the transportation tanks and the cost to pump it out has been studied with various combinations of stranded gas and compression applied. In the case of trucks, the total volume of stranded gas is not large, however, in very large ocean going vessels, the amount of gas stranded by contemporary methods can be very large.
Another problem associated with conventional methods of transportation are nefarious thermal issues. If the receiving tank pressure is zero and the transportation tank pressure is 3,000 p.s.i., for example, the instantaneous temperature drop upon opening the valve would be 84 degrees K or 151 degrees F., with very bad consequences if there was any water or foreign gases or liquids in the transportation tank. In addition to substantial thermal risks, the 3000 p.s.i. on the transportation tank and 0 p.s.i. in the receiving tank will average out to be 1500 p.s.i. in both tanks, with half of the gas being delivered. At that point gas compressors would be employed with more and more time and money spent as the percentage of the transported gas is transferred, as was also indicated above.